Mrcfile

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* Fix opening of large files in numpy 2.0 (in general this time, not just for
mmap)
* Raise errors if data or extended header arrays are too large for their
dimensions to be stored in the header
* Validate header statistics only if they are set

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* Fix opening of very large files with mmap in numpy 2.0 (thanks to Hanjin
Liu)

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* Add support for numpy 2.0

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* Add new 'indexed_extended_header' attribute to access the indexed part of
the extended header if it exists (thanks to David Waterman). Note that this
introduces a change in behaviour: code that used the 'extended_header'
attribute directly to access items in FEI1- and FEI2-type extended headers
will need to be updated to use 'indexed_extended_header' instead.
* Add support for numpy 1.24, 1.25 and 1.26 and Python 3.12

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* Add helper functions for working with header labels.
* Update header statistics calculation to use float32 instead of float64. This
is about twice as fast and uses less memory, but does make the calculations
slightly less accurate and will overflow (to 'inf') if the data array
contains very large values (larger than approximately 1e19).